This application is based on an application No.2000-140903 filed in Japan, the content of which is hereby incorporated by reference.
(1) Field of the Invention
The present invention relates to a high pressure discharge lamp, and particularity to improvements in an electrode for use in a high pressure discharge lamp of a short-arc type in which the distance between electrodes is made shorter than a conventional lamp.
(2) Related Art
In recent years, there has been active development for projection type image display apparatuses such as liquid crystal projectors. Such projection type image display apparatuses require high intensity light sources close to a point light source. As this kind of light source, short-arc high pressure discharge lamps, such as short-arc super-high pressure mercury lamps or short-arc metal-halide lamps are typically used.
These short-arc super-high pressure mercury lamps and short-arc metal-halide lamps conventionally used in the projection type image display apparatuses have employed tungsten electrodes with the same construction as those used in long-arc high pressure discharge lamps for general lighting. FIG. 1 shows an example of such a conventional electrode. As shown in the figure, an electrode 901 is usually composed of an electrode rod 902 made of tungsten and a coil 903 made of a tungsten wire having a narrow wire diameter.
However, such a short-arc high pressure discharge lamp that employs the electrode 901 has the following disadvantage. When the high pressure discharge lamp is lit, a tip of the electrode 901 is excessively heated, so that tungsten that forms the electrode 901 melts and evaporates. This blackens an inner surface of an arc tube, and also, deforms and wears the tip of the electrode 901. Accordingly, the high pressure discharge lamp cannot have a longer life. To overcome this disadvantage, various techniques have been devised aiming at improving electrodes.
As one technique for lengthening a life of a high pressure discharge lamp, an electrode that has been subjected to a fusing process so that the tip of the electrode is substantially shaped into a semi-sphere (such an electrode whose tip forms a semi-sphere is hereafter referred to as an xe2x80x9cimproved electrodexe2x80x9d) has been proposed. Japanese Patent No. 2820864 and Japanese Laid-Open Patent Application No. H10-92377 disclose techniques relating to such an improved electrode. FIGS. 2A and 2B are drawings for explaining a conventional improved electrode. An electrode 911 shown in the figures is produced in the following way. A coil 913 made of a tungsten wire is fixed around an electrode rod 912 made of tungsten as shown in FIG. 2A. The tip of the electrode rod 912 and a portion of the coil 913 are subjected to a fusing process with an electric discharge method to form an electrode tip 914 which is substantially a semi-sphere as shown in FIG. 2B.
The electrode tip 914 features the electrode 911. Since the electrode tip 914 has been made by fusing the portion of the coil 913, the heat generated in the electrode tip 914 at lighting is transferred rapidly to the coil 913, lowering the temperature of the electrode tip 914. This prevents tungsten from melting and evaporating, and further prevents the inner surface of the arc tube from being blackened and the end of the electrode 911 from being deformed and worn out. Accordingly, a life of a lamp employing the electrode 911 can be lengthened.
In recent development of short-arc high pressure discharge lamps for use in projection type image display apparatuses, two goals have been set. The two goals are (1) to improve luminance on a screen and (2) to provide varieties of lamps having the lamp power input approximately in the range of 50 to 400W, in view of keeping up with the recent trend towards diverse projection type image display apparatuses varying in area of a screen to be normally employed. As one approach to achieve goal (1), particularity to improve efficiency for light utilization when a reflective mirror is used in combination, the distance between electrodes xe2x80x9cDexe2x80x9d is shortened from the conventional distance of above 1.5 mm but not more than 2.5 mm, to the distance of 1.5 mm or shorter. Although active development has been made to provide short-arc high pressure discharge lamps in which the distance between electrodes is shorter than conventional lamps, mainly two problems lie ahead. One is that the shorter distance between electrodes leads to more serious deformation and wear of the electrode tip, accelerating the blackening of the inner surface of the arc tube. Due to this, the lamp life is shortened. The other unique problem that has become distinct is that an arc jump phenomenon occurs more frequently.
The following explains the arc jump phenomenon, with reference to FIG. 3. As shown in the figure, the arc jump phenomenon is a phenomenon where a luminescent spot of an electrode (a spot where an electronic current is discharged when the cathode is impressed) that is initially formed around the center of the electrode tip in a steady lighting state, changes its position in disorder with aging of the lamp. When the arc jump phenomenon occurs, a discharge arc deviates from the optical axis of a lamp unit in which a reflective mirror is incorporated. This causes significant fluctuation of luminance on a screen illuminated with the lamp unit.
As described above, for developing short-arc high pressure discharge lamps in which the distance between electrodes is made shorter than conventional lamps, two particular technical subjects are to be addressed. The two subjects are (1) to accomplish a lamp life of at least 3000 hours and (2) to provide high quality lamp characteristics for suppressing luminance fluctuation on the screen caused by an occurrence of the arc jump phenomenon.
In view of the above subjects, the object of the present invention is to provide effective techniques for realizing a long life of at least 3000 hours of a high quality high pressure discharge lamp of the short-arc type in which the distance between electrodes is shorter than a conventional lamp, and for suppressing luminance fluctuation on a screen illuminated using the high pressure discharge lamp.
The above object can be achieved by a high pressure discharge lamp, including: an arc tube having sealing parts at both ends thereof and a discharge space formed in the arc tube; two electrodes extending respectively from the sealing parts into the discharge space, so as to face each other with a predetermined distance xe2x80x9cDexe2x80x9d therebetween, each electrode being composed of an electrode rod and a coil wound around an end of the electrode rod; and mercury being filled in the discharge space in such a manner that a mercury vapor pressure therein in a steady lighting state is in the range of 15 MPa to 35 MPa inclusive; wherein the distance xe2x80x9cDexe2x80x9d is in the range of 0.5 mm to 1.5 mm inclusive, a tip of each electrode has been formed into a semi-sphere by fusing the electrode rod and the coil, and a thickness xe2x80x9cdexe2x80x9d and a diameter xe2x80x9cxcfx86exe2x80x9d of the semi-sphere are each in a range predetermined in correspondence with a power input of the high pressure discharge lamp.
The above arrangements have taught that a short-arc high pressure discharge lamp having various lamp power input (W) in which the distance between electrodes is shorter than a conventional lamp can realize a long life and can effectively suppress the arc jump phenomenon.